Rainforests

• The rainforest is incredibly complex• The rainforest is incredibly

productive• The rainforest cycles nutrients

incredibly quickly• The rainforest soils are incredibly

fragile

Rainforests

• Productivity: the amount of solar radiation, sunlight, converted by plants into complex molecules such as sugars

• This is accomplished through photosynthesis– CO2 + H2O + sunlight sugar +

O2

Rainforests

Rainforests

• Oxygen production very important both in ecological and evolutionary time

• E.g. oxygen-free to 21% O2

• E.g. rainforest – (2x) temperate forest– (4-5x) grassland

Rainforests

• NPP vs. GPP• GPP very hard to estimate, typically

NPP• Despite high rates of respiration (50-

60% in maintenance), highest NPP of all terrestrial systems

• So what…

Rainforests

• Conversion to less productive systems

• As rainforests are replaced by other systems (predominately agriculture), all are much less productive!– E.g. 40% of world’s NPP co-opted

Rainforests

• Leaf-area Index (LAI) is the leaf area above a square meter of forest floor.

• In NH, LAI is almost 6, BCI almost 8• In Caatinga, 5.1, and in rainforest, a

range of 10.6-22.4 (soil conditions)• The relationship between LAI and

understory cover is inversely related…

Rainforests

• Tropical growing season is year-round compared to temperate areas…

• Easy to suggest comparable on a per unit time metric

• Answer is unclear• But, some studies suggest tropical

growth is significantly greater than temperate growth (an order or magnitude greater)

Rainforests

• Growth: what influences it??• Water• Light

• CO2

• Minerals• Tropics do well in first 3, and plants have

done relatively well in adapting to poor soil

• Why??

Nutrient Cycling

• Earth is a closed system• Consequently, atoms present in dead

tissue must be reacquired, recycled back into the living world (or tissues)

• The recycling is in the form of decomposition

Nutrient Cycling

• Consider a unit of ‘rainforest unit of energy’ fixed during net primary productivity can move in one of two major directions: either it can be consumed as part of living tissue (caterpillar), or it can remain as part of the leaf until it drops and enters the decomposer food web

Nutrient Cycling

• Have you been to the tropics?

• Which route do you think the majority of energy takes? Why?

Nutrient Cycling

• Who are the players in the decomposer food web?– Fungi and bacteria– But also slime molds, actinomycetes,

algae, arthropods, earthworms, protozoans

• Having said that, our understanding of how the decomposer (microbial) food web works is relatively poor

Nutrient Cycling

• It may be the biodiversity in soil community rivals that of the canopy, but there are relatively few studies on the pathways and extent of this community

Nutrient Cycling • Organisms facilitate a process called

‘humification’, in which complex soil organic matter is maintained at the interface between the tree roots and soil.

• Humus is important in forming colloids that cement soil particles, in helping aerate the soil, in possessing a negative charge, and important aid in retaining critical minerals, and in overall conservation of nutrients

Nutrient Cycling

• The soil is an important ‘storage’ bank for essential minerals – E.g. nitrogen, calcium, magnesium,

phosphorus, and potassium

• Nutrient cycling is often termed ‘biogeochemical cycling’ because of the process moving things in between the living (bios) and non-living (geos)

Nutrient Cycling

• The rate of biogeochemical cycling is strongly influenced by temperature and rainfall

• Since minerals are always taken up through roots via water, the uptake of water is essential the uptake of minerals as well. But evaporation can be a mixed blessing. Plants can lose too much water when subjected to constant high temperature.

Nutrient Cycling

• Plants can lose too much water when subjected to constant high temperature. Many tropical plants retard evaporative water loss both by closing their stomata and by producing waxy leaves.

Leaching

• Water can wash essential minerals and other chemicals from leaves, a process called leaching. Leaching can be especially sever in areas subject o frequent heavy rains.

• A defense against this is the thick waxy coating that aid in retarding water loss as well as deterring herbivores and fungi

Leaching

• Rainfall also leaches minerals from the soil, washing them down into the deeper soil layers.

• Clay particles and humus have negative electrostatic charges tat attract mineral with positive charges such as calcium and potassium.

• Because H2O adds a positive H+ to the soil, these elements get washed deep into the soil or may wash out of the soil completely

Leaching

• Consequently in the tropics, the combination of high temperature and heavy rainfall can often result in much leaching and strongly acidic soils.

• Typical Amazon soils (up to 75%) are frequently mineral-poor, high in clay, acidic, and low in available phosphorus

Leaching

• As a result, in tropical forests most of the rapidly cycling minerals are in the living plants, the biomass.

• In the temperate zone, minerals are more equally distributed between the vegetation and soil bank.

Mycorrhizae

• Most plants in the world exhibit an intimate, mutualistic association between tree roots and a diverse group of fungi collectively termed mycorrhizae

• Although located in tree roots and appearing parasitic, they are essential in mineral uptake from the forest litter.

Rapid Recycling

• Walk through a rainforest and you’ll be surprised how little litter there is (think deciduous or more, coniferous)

• Rainforest have the same processes, they just occur much more rapidly

• E.g. One study estimates 80% of total leaf matter in an Amazon rainforest is annually returned to the soil.

Rainforest Soils

• Despite previous statements, generalizations of tropical soils should be made cautiously

• Eastern and central Amazon Basin, soils are very old and mineral-poor (oligotrophic), while in other regions, such as volcanic areas of Costa Rica or much of the Andes, soils are young and mineral-rich (eutrophic)

Rainforest Soils

• Soil characteristics vary regionally (and quite a bit) because soil is the product of several factors: climate, vegetation, topographic position, parent material, and soil age

• Much of the humid tropics falls into one of three categories: ultiols, oxisols, or alfisols, all of which are reddish to yellowish in color

Rainforest Soils

• Ultisols: well-weathered (leached)• Oxisols: deeply weathered, old, acidic,

and found on well-drained soils (generally occurring on old geologic formations). Widespread and world wide

• Alfisols: common in the subhumid and semiarid tropics and are closer to a neutral pH (though still acidic), which less overall leaching than oxisols

Rainforest Soils

• However, not all tropical soils are old or heavily weathered or infertile

• Perhaps 15% of moist tropical soils are situated on soils of at least moderate fertility. Soils generated from deposits during the flood cycle or from recent volcanic activity typify these categories

Rainforest Soils

• A general pattern in the tropics is that heat and heavy moisture input cause the formation of oxides of iron and aluminum (neither of which is useful to plants), yielding a reddish color

• Clay content is high…be leery of wet trails and mountain roads

Rainforest Soils

Rainforest Soils

Rainforest Soils

• At the extreme, one can encounter laterization, the combined effects of intensive erosion and heat acting on soil

• If vegetation is removed and bare soil is exposed to extreme downpours and heat, it can bake into a bricklike substance, ruining it for future productivity (also utilized for bricks)

• In Amazonia, only 4% are at risk

Rainforest Soils

• However, even without this extreme, the attempt to farm the Amazon and other tropical forests have failed due to the quick loss of soil fertility

• Limited agriculture could be supported (soil similar to SE US)…

more on this later…

Rainforest Soilswhite soils

• In some parts of the Amazon Basin, white and sandy soils predominate, from the Brazilian and Guianan Shields

• These are extremely old (100’s of millions of years) and have lost their fertility

• However, relatively lush broadleaf rainforests grow on these soils

Rainforest Soils

• Oligotrophic soils support less lush and smaller (in height) forests

• Up to 26% of roots can be on the surface and root mats can be very thick

• Certain minerals (e.g. calcium and phosphorus) are extremely efficiently taken up by root mat and mycorrhizae (99.9%)

Rainforest Soils

• How might buttresses impact the nutrient cycling? Roots can spread widely at the surface

• Do you understand the value and need for a thin layer of forest humus with its mycorrhizal fungi

• Generalizations should be avoided as there are many variants as to what is truly ‘productive’ and what is ‘tight’

Nutrient Adaptations

• Some tropical plants have root systems that grow vertically upward, from the soil onto the stems of neighboring trees. These apogeotropic roots grow as fast as (5.6cm) in 72 hrs).

• Why?

Nutrient Adaptations

• Arrested Litter: many plants (i.e. palms, epiphytes) catch litter before it reaches the forest floor. This is one way in which plants in the upper canopy can obtain the entire suite of nutrients needed to grown and reproduce

Nitrogen Fixation

• Some plants, such as legumes, can take up gaseous nitrogen directly from the atmosphere and convert it to nitrate, a chemical form in which it can be used by the plant.

• How effective? One estimate 20kg of nitrogen fixed per ha per yr in the Amazon Basin (>3x from precip)

Tropical Rivers

• White sandy soils are usually drained by blackwater rivers

• Color derived by tannins, phenolics, and related compounds (humic matter)

• Not confined to tropics (e.g. bogs)• Poor soils contribute to such water…• Leaf production expensive, hence

concentrate defenses

Rainforest Gaps

Tropical Rivers

• In contrast to white, sandy soils, soils in places such as Puerto Rico, much of Costa Rica, and much of the Andes Mountains are not mineral-poor but mineral-rich. They are much younger, mostly volcanic in origin, high rainfall and temperatures.

• Much sediment leaches into the river, they are cloudy and termed ‘whitewater’

Tropical Rivers

Tropical Rivers

• A dramatic confluence of such water is the ‘wedding of the waters’ between the Rio Negro and Rio Brancho

• It lasts from 15-25km until the mixing is complete

Tropical Rivers

Tropical Rivers